1. Field of the Invention
The present invention relates to an auto-tensioner and, more particularly, to an auto-tensioner that incorporates a pulley elastically supporting a belt.
2. Description of the Prior Art
One example of a conventional auto-tensioner is disclosed in Japanese Utility Model Application Laid-Open Publication Number 5-27405. This auto-tensioner includes a fixed shaft and an oscillatory motion member which is rotatably supported on the fixed shaft so as to be eccentrically movable about the fixed shaft and which has a cylindrical base portion. A pulley is rotatably supported on a shaft portion of the oscillatory motion member which is in parallel with the fixed shaft. A spring presses the pulley onto a belt in order to apply tension to the belt. A sealed space is formed between the base portion of the oscillatory motion member and an outer circumferential surface of the fixed shaft; the sealed space is filled with viscous fluid. A partition wall member divides the sealed space into two spaces in the axial direction. A first engaging means is provided between the partition wall member and the fixed shaft while a second engaging means is provided between the partition wall member and the oscillatory motion member. A passage is formed on the partition wall member and communications between the two spaces of the sealed space and a check valve which is disposed in the passage. The check valve opens the passage when the pulley is moved by the elastic force of the spring and closes the passage when the pulley is moved against the elastic force of the spring. Either the first engaging means or the second engaging means is used to make the partition wall member engage the fixed shaft or the oscillatory motion member so that the partition wall member can move relative to the fixed shaft of the oscillatory motion member only in the axial direction. The other of the first engaging means and the second engaging means then includes an inclination portion inclined in the axial and the circumferential directions and an engaging projection engaged with the inclination portion. The other of the first and second engaging means further makes the partition wall member move in the axial direction. When the rotational force is applied to the oscillatory motion member, the oscillatory motion member rotates relative to the partition wall member. Alternatively, when the rotational forces is applied to the partition wall member, the partition wall member rotates relative to the fixed shaft.
When the oscillatory motion member moves following the vibration of the belt pressed by the pulley, the partition wall member is axially moved in the sealed space. The belt has a tendency to be strained when the pulley is moved against the elastic force of the spring, whereby the check valve keeps the passage of the partition wall member closed. As a result, a large resistance is generated on the partition wall member which must move in the sealed space filled with the viscous fluid and the partition wall member can move only gradually. Accordingly, the pulley gradually follows the movement of the belt whose tension suddenly increased. In the process, other portions of the belt are prevented from becoming excessively loose. However, when the belt does become loose rapidly, namely when the pulley is moved by the elastic force of the spring, the check valve is opened so that the two spaces of the sealed space communicate through the passage with each other and the resistance against the movement of the partition wall member in the sealed space becomes small. As a result, the oscillatory motion member is moved by the elastic force of the spring and the pulley compensates for and follows the looseness of the belt.
In the above-mentioned prior auto-tensioner, since the sealed space is filled with the viscous fluid, the pressure of the viscous fluid in the sealed space increases with changes in the volume of the viscous fluid due to a change in temperature. When this occurs, there is the danger that seal portions of the sealed space will eventually be destroyed.
Furthermore, when the check valve is closed, viscous fluid flows through either the engaging means or the second engaging means, allowing the partition wall member to move axially relative to the fixed shaft or the oscillatory motion member. Also, the partition wall member is moved slowly in the axial direction. However, since either the first engaging means or the second engaging means slides while receiving the load in the circumferential direction when the oscillatory motion member moves, the engaging means receiving the load will more easily wear down. Consequently, when the engaging means receiving the load does down, the amount of flowing viscous fluid will change and, therefore, the above-discussed characteristics of the auto-tensioner will change.